Many printed circuit boards are distributed with a solder resist coating being formed thereon. In this case, though most of the surfaces of the printed circuit boards are covered with the solder resist coating, no solder resist coating is present on an electrode terminal (land) in order to mount electronic components. Accordingly, the surface of the electrode terminal is likely to be oxidized during distribution and/or storage of the printed circuit boards. In view of the above, gold plating is sometimes applied on the surface of the electrode terminal of the printed circuit board in order to prevent the oxidation of the surface of the electrode terminal. However, the use of noble metal (gold) for the gold plating necessarily increases the production cost. Accordingly, instead of applying the gold plating, a protection coating is formed on surfaces of electrode terminals of printed circuit boards using a water-based organic solderability preservative (see, for instance, Patent Literature 1: JP 6-322551 A).
A typical water-based organic solderability preservative as disclosed in Patent Literature 1 entails a problem of crystallization of an imidazole compound contained in the water-based organic solderability preservative when the water-based organic solderability preservative is stored at a low temperature (e.g. 5 degrees C. or less). Thus, it is demanded for the water-based organic solderability preservative to be capable of being stored in a low-temperature environment (low-temperature stability).
Since the imidazole compound in the water-based organic solderability preservative is not or poorly soluble to water, the water-based organic solderability preservative is usually dissolved in water with an assistance of an organic acid such as acetic acid. Though the properties including the low-temperature stability of varieties of types of imidazole compounds and organic acids have been studied, no combination of the imidazole compound and the organic acid exhibiting sufficient low-temperature stability has been found. In order to improve the low-temperature stability of the water-based organic solderability preservative, an organic solvent such as alcohols in which imidazole compounds can be dissolved may be used. However, the use of the organic solvent such as alcohols in order to improve the low-temperature stability results in difficulty in the formation of the protection coating of the imidazole compound on the surface of the electrode terminal. As described above, it has been very difficult to improve the low-temperature stability while keeping the formation performance of the protection coating.